BackgroundVariations due to soil type, nutrient levels and topography are common within a field. For efficient management of a field, it is necessary to map or "zone" these variations. Each of these zones of variation then becomes a "management zone" where productivity can be maximized based on variable rate applications using differential GPS.

The sugarbeet industry has evolved from an emphasis on tonnage to recoverable sugar per acre. The St. Thomas area in North Dakota has historically been a high tonnage, but low sugar content growing area. Due to this fact the emphasis has obviously switched to raising higher quality, i.e., greater sugar content. Studies have shown that inappropriate nitrogen levels are detrimental to beet quality.

Extensive soil sampling and groundtruthing by researchers have shown that dark green sugarbeet tops, with greater biomass, left as residue add large amounts of nitrogen back to the soil for subsequent crops. Because of the nitrogen carry-over, nitrogen credits from the previous crop must be accounted for in determining appropriate rates of fertilizer applications for the current crop. Satellite imagery is useful for zoning based on green tops.

 Use of Data The accompanying figure shows a 74 acre field that was zoned based on NDVI from IKONOS imagery. Using the image, the field where sugarbeets were grown in 2000 was zoned for variable rate application of nitrogen for the 2001 wheat crop. Note the circle in the north end of the field, which is the rim of a pothole that is the driest area in the field and hence lighter tone and less nitrogen credits. Such fine details are obtainable from high-resolution imagery, resulting in better-defined management zones.

 Economic and Environmental Benefits As compared to the conventional method of grid-based zoning, use of high-resolution imagery resulted in a saving of $810 for the field. This method of zoning resulted in average fertilizer cost saving of $466 and an increased revenue (due to increased sugar content) of $6050 per year over two years. Thus the total saving/income worked out to $7327 or $99 per acre! Besides, the variable rate application resulted in applying 35% (6300 pounds) less nitrogen, benefiting the environment.